(a) State and proof the Castigliano's theorem. (b) Using Castigliano's second theorem, determine the vertical deflection at joint F of the truss shown in Fig 3. Members have the same cross-sectional area of 600 mm and E = 200 GPa. 100 kN B CV D A F 3 m 2 at 3m 6m Fig. 3: A truss structure